The B cell response to antigen is regulated by a variety of co-receptors that convey information to the B cell about the quality of the antigen and the status of the ongoing immune response. Our progress using high resolution live cell imaging to delineate the very early antigen driven events in B cell activation has provided a new context in which the impact of coreceptors can be evaluated. Using high resolution fluorescence resonance energy transfer (FRET) coupled with total internal reflection microscopy (TIRFM) and single molecule tracking we provided evidence for an ordered process that occurs within seconds to minutes of the BCR binding antigen. Antigen bound BCRs form immobile clusters that then grow in size by molecular trapping. The clusters perturb the local lipid environment causing lipid rafts to coalesce around the BCR clusters. As a consequence of the membrane perturbation the first kinase in the pathway, Lyn, that is tethered to the membrane by raft lipids is brought into close molecular proximity to the BCR clusters. Simultaneously, Lyn phosphorylates the Ig alpha beta cytoplasmic domain of the BCR and the Ig alpha beta chains undergo a conformational change from a closed to an open form. Syk is recruited to the phosphorylated BCR and the signaling cascades are triggered. We learned that the FcgammaRIIB is excluded from the growing BCR cluster and that CD19 is recruited to the cluster in a ligand independent fashion. During the coming year we will pursue a new approach to better understand how the CD19/CD21 complex and the FcgammaRIIB regulate B cell responses. We will use super-resolution imaging to describe, at the 10-50 nm level, the spatial relationship between the BCR, CD19 and FcgammaRIIB in resting human B cells and in B cells in which the BCR is ligated alone by Ag or FcgammaRIIB is ligated by immune complexes. We are also characterizing the spatial distribution of these receptors on naive human IgM expressing naive B cells and in IgG-expressing memory B cells. These studies should provide a detailed spatial map of the distribution of these receptors on B cell surfaces and insight into the mechanisms underlying their function.